diff --git a/docs/releases/unreleased.md b/docs/releases/unreleased.md
index b75bbe1..fa884a3 100644
--- a/docs/releases/unreleased.md
+++ b/docs/releases/unreleased.md
@@ -5,3 +5,7 @@
 - All successfully read license states exit 0. Configuration, request, decoding, and output failures exit nonzero. No token or database header is sent to `/health`, and no authentication verification is performed. Normal connection selection, timeout, TLS verification, and identity opt-out settings apply.
 
 License installation and key/file configuration remain server responsibilities. Existing `ping -o json` license pass-through is unchanged. Closes [arcli#34](https://github.com/Basekick-Labs/arcli/issues/34).
+
+### Fixed
+
+- Reject reserved `time` field and tag names, including letter-case variants, before sending JSON writes. Timestamp columns must use lowercase `time`; row timestamps continue to use `t`. This prevents invalid names from causing a partially applied batch on the server.
diff --git a/internal/client/msgpack_reserved_names_test.go b/internal/client/msgpack_reserved_names_test.go
new file mode 100644
index 0000000..54a42ea
--- /dev/null
+++ b/internal/client/msgpack_reserved_names_test.go
@@ -0,0 +1,54 @@
+package client
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+)
+
+func TestJSONToMsgPackRejectsReservedTimeNames(t *testing.T) {
+	for _, name := range []string{"time", "Time", "TIME", "tImE"} {
+		for _, kind := range []string{"fields", "tags", "columns"} {
+			if kind == "columns" && name == "time" {
+				continue // the timestamp column itself is valid
+			}
+			t.Run(kind+"/"+name, func(t *testing.T) {
+				var row string
+				switch kind {
+				case "fields":
+					row = fmt.Sprintf(`{"m":"cpu","fields":{%q:1}}`, name)
+				case "tags":
+					row = fmt.Sprintf(`{"m":"cpu","fields":{"value":1},"tags":{%q:"label"}}`, name)
+				case "columns":
+					row = fmt.Sprintf(`{"m":"cpu","columns":{%q:[1700000000],"value":[1]}}`, name)
+				}
+				for _, doc := range []string{
+					row,
+					fmt.Sprintf(`[{"m":"cpu","fields":{"value":1}},%s]`, row),
+					fmt.Sprintf(`{"batch":[{"m":"cpu","fields":{"value":1}},%s]}`, row),
+				} {
+					out, _, err := JSONToMsgPack(strings.NewReader(doc))
+					if err == nil || !strings.Contains(err.Error(), "reserved") {
+						t.Errorf("%s: error = %v; want a reserved timestamp name error", doc, err)
+					}
+					if len(out) != 0 {
+						t.Error("invalid document produced MessagePack bytes")
+					}
+				}
+			})
+		}
+	}
+}
+
+func TestJSONToMsgPackAcceptsTimestampAndOtherNames(t *testing.T) {
+	for _, doc := range []string{
+		`{"m":"cpu","t":1700000000,"fields":{"runtime":1,"time_value":2},"tags":{"timezone":"UTC"}}`,
+		`{"m":"cpu","columns":{"time":[1700000000],"value":[1]}}`,
+		`{"m":"cpu","fields":{"TİME":1,"ｔｉｍｅ":2},"tags":{"TİME":"a","ｔｉｍｅ":"b"}}`,
+		`{"m":"cpu","columns":{"TİME":[1],"ｔｉｍｅ":[2]}}`,
+	} {
+		if _, _, err := JSONToMsgPack(strings.NewReader(doc)); err != nil {
+			t.Errorf("valid names rejected: %v", err)
+		}
+	}
+}
diff --git a/internal/client/msgpackenc.go b/internal/client/msgpackenc.go
index 7595fd3..65b9262 100644
--- a/internal/client/msgpackenc.go
+++ b/internal/client/msgpackenc.go
@@ -250,6 +250,9 @@ func convertItem(m map[string]any, where string, sum *MsgPackDocSummary) (any, e
 		outCols := make(map[string]any, len(cm))
 		n := -1
 		for col, v := range cm {
+			if col != "time" && strings.EqualFold(col, "time") {
+				return nil, fmt.Errorf(`%s: column name %q is reserved for the timestamp; use "time"`, where, col)
+			}
 			arr, ok := v.([]any)
 			if !ok {
 				return nil, fmt.Errorf("%s: column %q must be an array (the server would drop it)", where, col)
@@ -284,6 +287,9 @@ func convertItem(m map[string]any, where string, sum *MsgPackDocSummary) (any, e
 	}
 	outFields := make(map[string]any, len(fields))
 	for k, v := range fields {
+		if strings.EqualFold(k, "time") {
+			return nil, fmt.Errorf(`%s: field name %q is reserved for the timestamp; use "t" for row timestamps`, where, k)
+		}
 		cv, err := convertScalar(v, fmt.Sprintf("%s: field %q", where, k), false)
 		if err != nil {
 			return nil, err
@@ -315,6 +321,9 @@ func convertItem(m map[string]any, where string, sum *MsgPackDocSummary) (any, e
 		}
 		outTags := make(map[string]any, len(tm))
 		for k, v := range tm {
+			if strings.EqualFold(k, "time") {
+				return nil, fmt.Errorf(`%s: tag name %q is reserved for the timestamp`, where, k)
+			}
 			switch tv := v.(type) {
 			case string:
 				outTags[k] = tv
diff --git a/internal/commands/write_reserved_names_test.go b/internal/commands/write_reserved_names_test.go
new file mode 100644
index 0000000..5c928fa
--- /dev/null
+++ b/internal/commands/write_reserved_names_test.go
@@ -0,0 +1,40 @@
+package commands
+
+import (
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"sync/atomic"
+	"testing"
+)
+
+func TestWriteJSONRejectsReservedNamesBeforeRequest(t *testing.T) {
+	var requests atomic.Int32
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		requests.Add(1)
+		_, _ = io.Copy(io.Discard, r.Body)
+		w.WriteHeader(http.StatusBadRequest)
+		_, _ = io.WriteString(w, `{"error":"partial write: 1 points dropped; the other points were written"}`)
+	}))
+	defer srv.Close()
+	writeTestConfig(t, srv.URL, "tok")
+	for _, bad := range []string{
+		`{"m":"cpu","fields":{"time":1}}`,
+		`{"m":"cpu","fields":{"value":1},"tags":{"TIME":"label"}}`,
+		`{"m":"cpu","columns":{"Time":[1700000000],"value":[1]}}`,
+	} {
+		cmd := newWriteCmd()
+		cmd.SetIn(strings.NewReader(`{"batch":[{"m":"cpu","fields":{"value":1}},` + bad + `]}`))
+		out, _, err := execCmd(t, cmd, "--format", "json")
+		if err == nil || !strings.Contains(err.Error(), "reserved") {
+			t.Errorf("error = %v; want local reserved-name validation", err)
+		}
+		if out != "" {
+			t.Errorf("invalid document printed success: %q", out)
+		}
+	}
+	if got := requests.Load(); got != 0 {
+		t.Errorf("invalid batches reached the write endpoint %d times", got)
+	}
+}